The membrane members for pneumatically or hydraulically driven membrane valve assemblies that are commercial use in these days can be classified roughly in one of two groups:
Elastomeric membranes (EPDM-membranes) that are fixed to the pressure spindle and the pressure piece, respectively, by means of a screwed connection and PA1 Polytetrafluor ethylene membranes (PTFE-membranes) including a support membrane member made of an elastomeric material that are fixed to the pressure spindle and the pressure piece, respectively, by means of a bayonet connection. PA1 FC (fail close), i.e. the force of a biased spring closes the valve; PA1 FO (fail open), i.e. the force of a biased spring opens the valve; and PA1 DA (double acting), i.e. the force of a biased spring either opens or closes the valve.
By providing a bayonet connection, as disclosed, for example, in German Published Patent Application DE-A1-30 00 510, it is avoided that the closure pressure is induced from the pressure spindle via the membrane pin, whereby an axial displacement path is provided between pressure spindle or pressure piece and membrane member. An unduly high pressure acting onto the membrane pin can lead to damage of the membrane member and/or the lower portion of the valve housing.
In a membrane valve assembly of the kind referred to above, the pressure piece cannot be removed without disassembling the drive unit for operating the valve assembly. As a result, replacing the membrane assembly by another one requires quite a high effort if such replacement has to be done later on.
In the case of pneumatically operated membrane valve assemblies, the correct positioning of the connecting pieces for pressurized air required for operating the valve assembly is of essential importance. The disadvantage of known membrane valve assemblies may be seen in the fact that they do not provide for a reliable positioning of the connecting pieces for pressurized air which usually are integrated into the top housing portion of the valve assembly. Moreover, the housing portion can unintentionally become loose due to the lack of a certain bias force in the threaded connection.